<style>
    /* (行内，id, 类，标签) */
    /* (0, 0, 2, 1) */
    .c1 .c2 div {
        color: red;
    }

    /* (0, 1, 0, 1) */
    div #box3 {
        color: green;
    }

    /* (0, 1, 0, 1) */
    #box1 div {
        color: blue;
    }
</style>


<div id="box1"
     class="c1">
    <div id="box2"
         class="c2">
        <div id="box3"
             class="c3">白日依山尽，黄河入海流。</div>
    </div>
</div>